Abstract
The densification behavior of transition metal diboride compounds was reviewed with emphasis on ZrB2 and HfB2. These compounds are considered ultra-high temperature ceramics because they have melting temperatures above 3000?C. Densification of transition metal diborides is difficult due to their strong covalent bonding, which results in extremely high melting temperatures and low self-diffusion coefficients. In addition, oxide impurities present on the surface of powder particles promotes coarsening, which further inhibits densification. Studies prior to the 1990s predominantly used hot pressing for densification. Those reports revealed densification mechanisms and identified that oxygen impurity contents below about 0.5 wt% were required for effective densification. Subsequent studies have employed advanced sintering methods such as spark plasma sintering and reactive hot pressing to produce materials with nearly full density and higher metallic purity. Further studies are needed to identify fundamental densification mechanisms and further improve the elevated temperature properties of transition metal diborides.
Highlights
Transition metal diborides (TMB2s) have been researched for many years as materials for use in extreme environments.[1,2,3,4,5,6,7] Several TMB2s, including TiB2, ZrB2, HfB2, and TaB2, are considered to be ultra-high temperature ceramics (UHTCs) because they have melting temperatures in excess of 3000°C
The purpose of this paper is to critically evaluate published studies of the densification behavior of nominally pure TMB2 ceramics with the focus on ZrB2 and HfB2
Transition metal diboride ceramics can be densified by methods including hot pressing, pressureless sintering, and reaction-based processes
Summary
Transition metal diborides (TMB2s) have been researched for many years as materials for use in extreme environments.[1,2,3,4,5,6,7] Several TMB2s, including TiB2, ZrB2, HfB2, and TaB2, are considered to be ultra-high temperature ceramics (UHTCs) because they have melting temperatures in excess of 3000°C. Some of the potential applications typically mentioned for TMB2s include hypersonic aerospace vehicles, rocket motors, scramjet engines, lightweight armor, high-speed cutting tools, refractories for molten metal contact applications, plasma-facing materials for nuclear fusion reactors, and fuel forms for advanced nuclear fission reactors.[5,14,15,16,17,18,19,20,21,22] The same characteristics that give TMB2s their remarkably high melting temperatures and hardness values make TMB2s difficult to densify. Ceramics containing significant additions of SiC, MoSi2, or other second phases are intentionally excluded from consideration in this manuscript
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